docs: def append_formula(self, formula, no_return=True):
          """
              This method can be used to add a given list of clauses into the
              solver.
  
              :param formula: a list of clauses.
              :param no_return: check solver's internal formula and return the
                  result, if set to ``False``.
  
              :type formula: iterable(iterable(int))
              :type no_return: bool
  
              The ``no_return`` argument is set to ``True`` by default.
  
              :rtype: bool if ``no_return`` is set to ``False``.
  
              .. code-block:: python
  
                  >>> cnf = CNF()
                  ... # assume the formula contains clauses
                  >>> s = Solver()
                  >>> s.append_formula(cnf.clauses, no_return=False)
                  True
          """
  
          if self.solver:
              res = self.solver.append_formula(formula, no_return)
              if not no_return:
                  return res